1. Field of the Invention
This invention generally relates to subsea drilling system and method, and in particular to a system and method adapted for use with a rotating control device (RCD) to sealably control fluid flow in a riser.
2. Description of Related Art
Marine risers extending from a wellhead fixed on the floor of an ocean have been used to circulate drilling fluid back to a structure or rig. The riser must be large enough in internal diameter to accommodate the largest bit and pipe that will be used in drilling a borehole into the floor of the ocean.
An example of a marine riser and some of the associated drilling components is proposed in U.S. Pat. Nos. 4,626,135 and 7,258,171. As shown in FIG. 1 of the '171 patent, since the riser R is fixedly connected between a floating structure or rig S and the wellhead W, a conventional slip or telescopic joint SJ, comprising an outer barrel OB and an inner barrel IB with a pressure seal therebetween, is used to compensate for the relative vertical movement or heave between the floating rig and the fixed riser. A diverter D has been connected between the top inner barrel IB of the slip joint SJ and the floating structure or rig S to control gas accumulations in the marine riser R or low pressure formation gas from venting to the rig floor F. A ball joint BJ above the diverter D compensates for other relative movement (horizontal and rotational) or pitch and roll of the floating structure S and the fixed riser R.
The diverter D can use a rigid diverter line DL extending radially outwardly from the side of the diverter housing to communicate drilling fluid or mud from the riser R to a choke manifold CM, shale shaker SS or other drilling fluid receiving device. Above the diverter D is the rigid flow line RF, configured to communicate with the mud pit MP. If the drilling fluid is open to atmospheric pressure at the bell-nipple in the rig floor F, the desired drilling fluid receiving device must be limited by an equal height or level on the structure S or, if desired, pumped by a pump to a higher level. While the shale shaker SS and mud pits MP are shown schematically in FIG. 1 of the '171 patent, if a bell-nipple were at the rig floor F level and the mud return system was under minimal operating pressure, these fluid receiving devices may have to be located at a level below the rig floor F for proper operation. Since the choke manifold CM and separator MB are used when the well is circulated under pressure, they do not need to be below the bell nipple.
As also shown in FIG. 1 of the '171 patent, a conventional flexible choke line CL has been configured to communicate with choke manifold CM. The drilling fluid then can flow from the choke manifold CM to a mud-gas buster or separator MB and a flare line (not shown). The drilling fluid can then be discharged to a shale shaker SS, and mud pits MP. In addition to a choke line CL and kill line KL, a booster line BL can be used.
In the past, when drilling in deepwater with a marine riser, the riser has not been pressurized by mechanical devices during normal operations. The only pressure induced by the rig operator and contained by the riser is that generated by the density of the drilling mud held in the riser (hydrostatic pressure). During some operations, gas can unintentionally enter the riser from the wellbore. If this happens, the gas will move up the riser and expand. As the gas expands, it will displace mud, and the riser will “unload.” This unloading process can be quite violent and can pose a significant fire risk when gas reaches the surface of the floating structure via the bell-nipple at the rig floor F. As discussed above, the riser diverter D, as shown in FIG. 1 of the '171 patent, is intended to convey this mud and gas away from the rig floor F when activated. However, diverters are not used during normal drilling operations and are generally only activated when indications of gas in the riser are observed. The '135 patent proposed a gas handler annular blowout preventer GH, such as shown in FIG. 1 of the '171 patent, to be installed in the riser R below the riser slip joint SJ. Like the conventional diverter D, the gas handler annular blowout preventer GH is activated only when needed, but instead of simply providing a safe flow path for mud and gas away from the rig floor F, the gas handler annular blowout provider GH can be used to hold limited pressure on the riser R and control the riser unloading process. An auxiliary choke line ACL is used to circulate mud from the riser R via the gas handler annular blowout preventer GH to a choke manifold CM on the rig.
More recently, the advantages of using underbalanced drilling, particularly in mature geological deepwater environments, have become known. Deepwater is generally considered to be between 3,000 to 7,500 feet deep and ultra deepwater is generally considered to be 7,500 to 10,000 feet deep. Rotating control heads or devices (RCD's), such as disclosed in U.S. Pat. No. 5,662,181, have provided a dependable seal between a rotating pipe and the riser while drilling operations are being conducted. U.S. Pat. No. 6,138,774, entitled “Method and Apparatus for Drilling a Borehole into a Subsea Abnormal Pore Pressure Environment,” proposes the use of a RCD for overbalanced drilling of a borehole through subsea geological formations. That is, the fluid pressure inside of the borehole is maintained equal to or greater than the pore pressure in the surrounding geological formations using a fluid that is of insufficient density to generate a borehole pressure greater than the surrounding geological formation's pore pressures without pressurization of the borehole fluid. U.S. Pat. No. 6,263,982 proposes an underbalanced drilling concept of using a RCD to seal a marine riser while drilling in the floor of an ocean using a rotatable pipe from a floating structure. Additionally, U.S. Provisional Application No. 60/122,350, filed Mar. 2, 1999, entitled “Concepts for the Application of Rotating Control Head Technology to Deepwater Drilling Operations” proposes use of a RCD in deepwater drilling.
It has also been known in the past to use a dual density mud system to control formations exposed in the open borehole. See Feasibility Study of a Dual Density Mud System for Deepwater Drilling Operations by Clovis A. Lopes and Adam T. Bourgoyne, Jr., © 1997 Offshore Technology Conference. As a high density mud is circulated from the ocean floor back to the rig, gas is proposed in this May of 1997 paper to be injected into the mud column at or near the ocean floor to lower the mud density. However, hydrostatic control of abnormal formation pressure is proposed to be maintained by a weighted mud system that is not gas-cut below the ocean floor. Such a dual density mud system is proposed to reduce drilling costs by reducing the number of casing strings required to drill the well and by reducing the diameter requirements of the marine riser and subsea blowout preventers. This dual density mud system is similar to a mud nitrification system, where nitrogen is used to lower mud density, in that formation fluid is not necessarily produced during the drilling process.
As proposed in U.S. Pat. No. 4,813,495, a subsea RCD has been proposed as an alternative to the conventional drilling system and method when used in conjunction with a subsea pump that returns the drilling fluid to a drilling vessel. Since the drilling fluid is returned to the drilling vessel, a fluid with additives may economically be used for continuous drilling operations. ('495 patent, col. 6, ln. 15 to col. 7, ln. 24) Therefore, the '495 patent moves the base line for measuring pressure gradient from the sea surface to the mudline of the sea floor ('495 patent, col. 1, lns. 31-34). This change in positioning of the base line removes the weight of the drilling fluid or hydrostatic pressure contained in a conventional riser from the formation. This objective is achieved by taking the fluid or mud returns at the mudline and pumping them to the surface rather than requiring the mud returns to be forced upward through the riser by the downward pressure of the mud column ('495 patent, col. 1, lns. 35-40).
Conventional RCD assemblies have been sealed with a subsea housing active sealing mechanisms in the subsea housing. Additionally, conventional RCD assemblies, such as proposed by U.S. Pat. No. 6,230,824, have used powered latching mechanisms in the subsea housing to position the RCD.
Additionally, the use of a RCD assembly in a dual-density drilling operation can incur problems caused by excess pressure in either one of the two fluids. The ability to relieve excess pressure in either fluid would provide safety and environmental improvements. For example, if a return line to a subsea mud pump plugs while mud is being pumped into the borehole, an overpressure situation could cause a blowout of the borehole. Because dual-density drilling can involve varying pressure differentials, an adjustable overpressure relief technique has been desired.
Another problem with conventional drilling techniques is that moving of a RCD within the marine riser by tripping in hole (TIH) or pulling out of hole (POOH) can cause undesirable surging or swabbing effects, respectively, within the well. Further, in the case of problems within the well, a desirable mechanism should provide a “fail safe” feature to allow removal of the RCD upon application of a predetermined force.
U.S. Pat. Nos. 6,470,975; 7,159,669; and 7,258,171 propose positioning an RCD assembly in a housing positioned in a marine riser. In the '171 patent, a system and method are disclosed for drilling in the floor of an ocean using a rotatable pipe. The system uses a RCD with a bearing assembly and a holding member for removably positioning the bearing assembly in a subsea housing. The bearing assembly is sealed with the subsea housing by a seal, providing a barrier between two different fluid densities. The holding member resists movement of the bearing assembly relative to the subsea housing. The bearing assembly is proposed to be connected with the subsea housing above or below the seal.
In one embodiment of the '171 patent, the holding member rotationally engages and disengages a passive internal formation of the subsea housing. In another embodiment of the '171 patent, the holding member engages the internal formation, disposed between two spaced apart side openings in the subsea housing, without regard to the rotational position of the holding member. The holding member of the '171 patent is configured to release at predetermined force.
The holding member assembly of the '171 patent provides an internal housing concentric with an extendible portion. When the extendible portion extends, an upper portion of the internal housing is proposed to move toward a lower portion of the internal housing to extrude an elastomer disposed between the upper and lower portions to seal the holding member assembly with the subsea housing. The extendible portion is proposed to be dogged to the upper portion or the lower portion of the internal housing depending on the position of the extendible portion.
As further proposed in the '171 patent, a running tool is used for moving the rotating control head assembly with the subsea housing and is also used to remotely engage the holding member with the subsea housing.
Latching assemblies have been proposed in the past for positioning an RCD. U.S. Pat. No. 7,487,837 proposes a latch assembly for use with a riser for positioning an RCD. Pub. No. US 2006/0144622 A1 proposes a latching system to latch an RCD to a housing and active seals. Pub. No. US 2008/0210471 A1 proposes a docking station housing positioned above the surface of the water for latching with an RCD.
The above discussed U.S. Pat. Nos. 4,626,135; 4,813,495; 5,662,181; 6,138,774; 6,230,824; 6,263,982; 6,470,975; 7,159,669; 7,258,171; and 7,487,837; and Pub. Nos. US 2006/0144622 A1 and 2008/0210471 A1; and U.S. Provisional Application No. 60/122,350, filed Mar. 2, 1999, entitled “Concepts for the Application of Rotating Control Head Technology to Deepwater Drilling Operations” are all hereby incorporated by reference for all purposes in their entirety. The '181, '774, '982 and '171 patents, and the '622 and '471 publications are assigned to the assignee of the present invention.
In cases where reasonable amounts of gas and small amounts of oil and water are produced while drilling underbalanced for a small portion of the well, it would be desirable to use conventional rig equipment in combination with a RCD, to control the pressure applied to the well while drilling. Therefore, a system and method for sealing with a subsea housing including, but not limited to, a blowout preventer while drilling in deepwater or ultra deepwater that would allow a quick rig-up and release using conventional pressure containment equipment would be desirable. In particular, a system that provides sealing of the riser at any predetermined location, or, alternatively, is capable of sealing the blowout preventer while rotating the pipe, where the seal could be relatively quickly installed, and quickly removed, would be desirable.